# [x, y]           product sum = x + y
# [x, [y, z]]      product sum = x + 2 * (y + z)
# [x, [y, [z, q]]] product sum = x + 2 * (y + 3 * (z + q))

# [1, 2]      product sum = 3
# [1, [2, 3]] product sum = 1 + 2 * (2 + 3) = 11